Primary Mechanism of Goitre Formation in Iodine Deficiency
The correct answer is C: Chronic TSH stimulation is the primary mechanism of goitre formation in a euthyroid woman from an iodine-deficient region with diffuse goitre.
Pathophysiological Mechanism
In iodine deficiency, the thyroid gland experiences chronic TSH stimulation as a compensatory response to maintain euthyroid status, which directly causes thyroid enlargement (goitre). 1
The mechanism operates through the following sequence:
TSH regulates all steps of thyroid hormone biosynthesis, including iodide uptake, oxidation and organification of iodide, and secretion of T4 and T3 into circulation 1, 2
When iodine is deficient, the thyroid cannot produce adequate thyroid hormone despite increased TSH-driven activity, leading to compensatory thyroid gland enlargement to maximize iodine uptake and recycling 1
The patient remains euthyroid because the increased thyroid activity and gland size compensate for low iodine availability, maintaining normal thyroid hormone production despite the deficiency 1
Why Other Options Are Incorrect
Option A (Autoimmune stimulation) is incorrect because this describes Graves' disease, not iodine-deficiency goitre 3
Option B (Excess thyroid hormone synthesis) is incorrect because the patient is euthyroid, not hyperthyroid, and iodine deficiency actually impairs hormone synthesis 1
Option D (Thyroid hormone resistance) is incorrect because this is a genetic disorder unrelated to iodine deficiency 2
Option E (Increased thyroglobulin destruction) is incorrect because goitrogenesis involves increased thyroid activity and growth, not destruction of thyroglobulin 1
Important Clinical Nuances
TSH levels may remain within normal range despite frank iodine deficiency, making TSH an insensitive indicator of iodine status 1, 4. This explains why the patient can have a goitre while remaining biochemically euthyroid.
The intrathyroidal iodine concentration is low in endemic goitre, but this is a secondary event rather than the primary cause - the fundamental driver is the chronic TSH stimulation attempting to compensate for inadequate iodine substrate 5
While some research suggests intrinsic thyroid factors may contribute to nodular goitre development 6, 5, the initial diffuse goitre formation in iodine-deficient regions is primarily TSH-mediated 1, 3